Let's see... my favorite threads in the QR/QQ to play are:
Palantir of Fortune
20 Questions on ME
The "is it" game
Various trivia games
In the Books, I had great enjoyment posting in the "Nebulous It" thread a while back. I liked following the "Canonicity" thread, though I never actually posted in it. Another thread I found interesting was the "Hope vs. Hopelessness" thread, which was before I registered but was interesting reading nonetheless. And, of course, the Chapter-by-Chapter discussions provide many enlightening thoughts and posts.
In ME Mirth, my favorite thread is probably the "Create Your Own Crazy Scene with Pics". Hilarious. "Trilogy Tuesday: What not to do" and some other thread that I can't seem to find or remember the name of...
And, in the Barrow-downs section, the "Know Yer Mates" is my current favorite.
Sorry, I'm too lazy to go find all of these threads and link them.